This invention relates to fuel cells and, in particular, to the prevention of electrolyte migration in these cells.
Fuel cells are well known in the art. These cells typically comprise porous anode and cathode electrodes which sandwich an electrolyte usually contained in a matrix. The electrode/matrix sandwich is situated between plates which define chambers for bringing fuel and oxidant process gases to the anode and cathode electrodes, respectively.
Cells formed in this manner are placed one on top of the other to form a fuel cell stack. The resultant fuel cell stack has end faces formed by the end faces of the individual cell components. These end faces communicate with manifolds which carry supply gases, both fuel and oxidant, to the stack as well as spent gases from the stack. To ensure gas-tight mating of the manifolds with the stack end faces, gaskets are typically interposed between the stack end faces and the manifold. These gaskets are usually porous.
In fuel cell stacks of the above type, it has been found that there is an undesirable migration of electrolyte through the stack. Thus, it has been found that the electrolyte from the individual cells migrates from the positive to negative end of the stack. This migration is driven by the stack voltage and causes the cells at the negative end of the stack to be flooded with electrolyte and the cells at the positive end of the stack to become depleted of electrolyte.
Such migration of electrolyte has been found to be most severe in fuel cell stacks employing molten carbonate fuel cells. In such stacks the migration can severely reduce stack performance and has been observed in stacks of far fewer cells than the several hundred cells contemplated for viable commercial power plants. Moreover, where the molten carbonate fuel cells employ a mixture of carbonates (e.g., K.sub.2 CO.sub.3 and Li.sub.2 CO.sub.3) as the electrolyte, there is a preferred movement of one of the carbonates (i.e., the K.sub.2 CO.sub.3) to the negative end of the stack. This redistribution of the electrolyte further reduces stack performance.
It is known that the migration of electrolyte in fuel cell stacks is due to shunt currents which promote ionic electrolyte flow. These shunt currents are dependent upon the stack voltage and resistance to electrolyte flow. The latter resistance, in turn, is determined by the resistance of the cell weet seal and the resistance of the above-discussed manifold sealing gasket. These resistances act in series, with the resistance of the sealing gasket playing the dominant role and being rate controlling.
A variety of techniques have been proposed for counteracting the electrolyte migration problem. One proposed technique is disclosed in U.S. Pat. No. 4,591,438 and contemplates the use of a higher than usual content of Li.sub.2 CO.sub.3 in the Li.sub.2 CO.sub.3 /K.sub.2 CO.sub.3 electrolyte to maintain a uniform electrolyte molar ratio along the length of the stack. A second proposed technique is described in U.S. Pat. No. 4,643,954. In this technique, a passageway is provided for returning excess electrolyte from the negative end of the stack, where electrolyte flooding occurs, to the positive end of the stack, where electrolyte is depleted. Wicks are provided in the ends of this passageway to promote communication with the cell components and the end of the passageway where there is excess electrolyte is heated to increase the electrolyte vapor pressure.
A further attempt at a solution to the electrolyte migration problem is disclosed in U.S. Pat. No. 4,7613,348. In this patent, reservoirs are provided at the negative and positive ends of the stack for receiving excess electrolyte and supplying depleted electrolyte, respectively. These reservoirs are developed by forming the manifold gasket to have end portions of increased volume and cross-section The mid-portion of the gasket, in turn, is of lesser volume and cross-section and formed from a fiberous ceramic strip which acts to retard migration of electrolyte from the positive to negative end of the stack.
The above techniques have offered some limited solution to the electrolyte migration problem. However, is not believed that they can sufficiently reduce migration to bring it within tolerable limits. Furthermore, the techniques are cumbersome and not easily implemented.
It is therefore an object of the present invention to provide an improved apparatus and method for reducing the electrolyte migration in fuel cells.
It is yet a further object of the present invention to provide a manifold gasket for molten carbonate fuel cells which is adapted to reduce electrolyte migration.
It is yet a further object of the present invention to provide a gasket of the type described in the previous objective which can be easily and readily implemented.